If the IRS is coming after you for not paying taxes on unreported income, it may be that someone used your Social Security Number on his W-4. According to the Treasury Department's Inspector General for Tax Administration, it happened as many as 1.2 million times in 2007.

What is worse, the full article on WebCPA says that the IRS lacks the procedures to identify this type of identity theft:

TIGTA assessed whether the IRS has procedures to effectively handle
collection issues related to ITINs [Individual Taxpayer Identification Number]. It found that the IRS lacks internal
guidelines for its employees to follow to assist either the taxpayer
whose wages are being attached or the legitimate holder of the Social
Security number (who may unknowingly be the victim of identity theft).

Just another way identity theft can bite you!! It appears there is no way to protect yourself against this one. Perhaps the credit agencies could detect this.